The present invention relates to the removal of an undesired material from the surface of a substrate with a desired material in place on the substrate while minimizing the loss of the desired material. It finds particular application in the etching, cleaning, or removal of silicon oxide and contaminant films from semiconductor surfaces or in topographic features of a semiconductor wafer. In particular, it relates to the removal of silicon oxides and other contaminants in a dry, gas-phase environment where ultraviolet (UV) light stimulation and a fluorine-containing molecular gas such as chlorine triflouride are used to etch different forms of silicon dioxide at similar rates without the significant generation of water as a reaction by-product.
In semiconductor device processing, oxides of silicon are used in many different forms for many applications. Dense, thermally grown oxides of silicon are typically used as the primary gate dielectric film in MOS (metal oxide-silicon) transistors. Steam grown thermal oxides are commonly used as a field oxidation dielectric layer. Doped oxides such as phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) are commonly used as inter-metal layer dielectrics because they can be easily planarized with an elevated temperature reflow process. Spin-on-glass (SOG) is also used in dielectric applications where planarization is critical. An SOG is a siloxane-type polymer in an organic solvent which is deposited in liquid form and then cured at elevated temperature to form a solid silicon oxide film.
During the processing of silicon based semiconductor devices, other types of oxide films may be formed as the result of exposure of silicon surfaces to chemical processing steps, or to the ambient environment. For instance, the well known RCA wet cleaning sequence is known to leave a 10-20 .ANG. "chemical" oxide on the surface. Exposure of a clean silicon surface to ambient atmosphere results in the growth of a 5-10 .ANG. "native" oxide. In many cases, these residual oxides are considered surface contaminants since they must be removed to reveal a pristine silicon surface to allow the formation of a high quality electrical interface. Interlayer metal contacts made through vias or "contact holes" in a dielectric layer such as BPSG are of high quality only when oxides and contaminants on the lower metal or polysilicon level are removed. Often the contamination in contact holes or on feature sidewalls which is the result of plasma or reactive ion etching processes may be comprised of a mixture of silicon oxides, silicides or oxides of metals, and organic contaminants.
Very often, it is necessary to remove a chemical or native oxide, or post-etch residue contamination from a pattern feature bottom or from an exposed wafer surface in the presence of one or more of the many other types of silicon oxides mentioned above. It has long been known that vapors of HF/water mixtures will etch various silicon oxide films. This technology has been studied and commercialized (U.S. Pat. Nos. 4,749,440 and 4,938,815). However, several limitations are sometimes encountered in the use of HF vapor phase etching of oxide films. These limitations can include the formation of non-volatile residues which must be rinsed away, and low etching rates for native and chemical silicon oxide films relative to doped silicon oxide films. In addition, water is generated as a reaction by-product, which can make the anhydrous HF processes difficult to perform controllably and repeatably. Water is in general among the most undesirable of chemical species to have present in a vacuum environment.
The relative rate of etching (selectivity) of the HF vapor etching processes to many different types of oxide films has also been studied. The results show that native, chemical, and thermal oxides are typically removed at rates 10 times slower than the removal rates of PSG and BPSG doped silicon oxides. This is problematic in several common processing circumstances. First of all, it is commonly necessary to clean native oxide and other contaminants from the bottom of contact holes formed in films of BPSG. Using the current vapor phase processes, several hundred angstroms of the BPSG are removed before the silicon oxides and contaminants in the bottom of the contact hole are removed. Etching a large quantity of BPSG is unfavorable and may leave undesirable residues. Second, it is common to use composite structures of different types of silicon oxide films. For instance, a BPSG layer sandwiched between two undoped silicon oxide layers is sometimes used as a dielectric film between metal layers. Cleaning of contact or other topographic features through this type of composite film with the current HF vapor technology causes enhanced lateral etching of the BPSG layer relative to the undoped silicon oxide layers. This results in an undercut profile which is difficult to fill with subsequent films without forming voids. For this case, a non-selective oxide removal process is most desirable, i.e., a process that etches native, chemical, and thermal oxides at nearly the same rate as doped oxides. Third, it is sometimes desirable to remove a thermal oxide film over a doped oxide without over-etching the doped oxide extensively.
Attempts have been made to address the limitations of the aqueous HF vapor technology described above by substituting alcohol vapor in place of water vapor in combination with the HF gaseous reactant. However, water is generated as a by-product of the process, leading to many of the same limitations as the aqueous HF vapor technologies. Furthermore, the use of HF with alcohol vapor gives very high removal rates of BPSG relative to native oxide contamination. Also, the use of HF with alcohol vapor in the presence of BPSG can still result in problematic residue formation.
Other attempts to remove silicon oxide films in a dry, gas-phase reaction environment have been made which do not utilize HF. The effluent from a plasma of nitrogen trifluoride (NF.sub.3) and hydrogen (H.sub.2) has been used to remove oxide films. Also, fluorine (F.sub.2) and hydrogen mixtures with UV illumination have been used to remove oxide films. The presence of hydrogen in the reaction chemistry still leads to the formation of water as a reaction by-product.
Previous work with ClF.sub.3 (U.S. Pat. No. 4,498,953) indicated that thermal oxide removal rates with ClF.sub.3 exposure were not measurable. It was, in fact, reported that silicon oxide was successfully used as mask material in the etching of silicon by ClF.sub.3. This work did not utilize UV illumination.